1. Field Of The Invention
This invention relates generally to battery power supply systems, such as used in vehicles, and more particularly relates to an electronic circuit for equalizing the voltage on two or more series connected storage batteries.
2. Description Of The Related Art
Many commercial vehicles, such as buses, utility trucks and tractors, are provided with on board electrical systems, including one or more batteries, for supplying electrical power to a variety of electrical loads, such as lights, specialized equipment and vehicle accessories. Some of these loads are motors, such as a starting motor for a large diesel engine, or for driving air conditioners, which must provide a high torque and therefore have a high power requirement. It is desirable to supply such high power loads at a voltage greater than 12 volts, such as 24 volts, in order to meet the high torque requirements without excessive current and consequent resistive losses. However, storage batteries for use on vehicles are conventionally manufactured with a standard 12-volt nominal battery voltage. Furthermore, lights, fare boxes, radios, transmissions and many accessories and other equipment for use on vehicles are also conventionally manufactured for use with a nominal 12 volt system.
Therefore, it is desirable to take advantage of the cost savings resulting from use of conventionally available 12 volts electrical equipment and 12 volt batteries on a vehicle while also providing a DC supply voltage greater than 12 volts in order to supply the larger loads, such as motors. This has traditionally been accomplished by utilizing two or more 12 volt batteries connected in series. The high power requirements can then be delivered at the sum voltage of the series batteries and yet power can be supplied to the 12 volt loads at the voltage of the 12 volt batteries. In this manner, the 12 volt loads can be supplied by connecting them in parallel to one of the 12 volt batteries, typically the battery which is connected to vehicle ground, and the higher voltage loads can be connected parallel to the entire set of series connected batteries.
Unfortunately, however, in the absence of additional circuitry, such connection causes the battery which is parallel to the 12 volt loads to become undercharged, drained, and reduced in voltage, while the other battery or batteries becomes overcharged and raised to an excessive voltage. This occurs because the alternator is connected parallel to the series connected batteries and consequently all charging current which is used to charge the battery supplying the 12 volt loads must also flow through the other 12 volt battery or batteries. Therefore, a series battery which does not supply the 12 volt loads becomes overcharged and, as a result, the grounded battery which supplies the 12 volt loads cannot be maintained in a charged condition at full voltage.
In order to correct this problem, battery equalizer circuits were developed and are the subject of my previous U.S. Pat. Nos., 4,479,083 and 5,528,122, both of which are herein incorporated by reference. The battery equalizer circuits of those patents are three terminal devices which are connected to the three terminals of two series connected batteries. They cause two series connected batteries of equal voltage to contribute equally to the current supplied to the 12 volt loads as if the two batteries were connected in parallel. The battery equalizer extends battery life by maintaining the voltages of the two batteries equal because the equalizer holds the voltage, at the intermediate terminal between the two batteries, at a voltage which is equal to one-half the sum of the two battery voltages. Therefore, neither battery becomes significantly overcharged or undercharged.
Battery equalizer circuits in the prior art, as well as the present invention, utilize a DC to DC converter circuit. The prior art teaches many varieties of DC--DC converter circuits to which the present invention can be applied. For use in a battery equalizer, a DC--DC converter transfers energy from one battery to the other. This is accomplished by using a reactive, energy transferring circuit element, which is switched in alternating connection to each battery by transistors used as switches. In some DC--DC converters the energy is received from the higher voltage battery and intermediately stored in one or two inductors or capacitors of the energy transferring circuit element during one-half cycle, and then is transferred to the lower voltage battery during the other half cycle. In others, the energy transferring circuit is a transformer which transfers energy by inductive coupling during each half cycle from the higher voltage battery to the lower voltage battery. The DC--DC converter portion of the circuit is not itself the present invention. Many varieties of DC--DC converters are shown in the prior art and the present invention is applicable to most of them.
The prior art also teaches control circuits, particularly pulse width modulation circuits, for controlling the transistor switches. The transistor switches are alternatingly switched on and off with a pulse width or duty cycle which is modulated as an increasing function of a feedback error signal. That error signal is the difference between the desired battery voltage and the actual battery voltage. Typically, the error signal is the difference between one-half the sum of the voltages of two series connected batteries and the voltage of the grounded one of the batteries.
Many common, popular DC to DC converter circuits include a transformer as the energy transferring circuit element. The transformer is switched in alternate connections to the batteries by means of transistor switches. This switching causes substantial transient variations, resulting particularly from a need which arises during higher current operation of the equalizer with a 50% duty cycle (the energy transferring circuit element being connected to the batteries by the transistor switches substantially 100% of the cycle, 50% in each alternate connection). The need is for a time interval between alternate connections of the transformer to the battery, during which all transistor switches are turned off in order to avoid any possibility of a momentary, short circuit through the transistor switches. These switching transients generate spurious noise, which can interfere with electronic instruments and equipment on board a vehicle. Consequently, it is necessary that filtering be provided.
It is an object and feature of the present invention to provide a circuit which maintains an essentially constant current through its main current conducting branches for all switching states of the DC--DC converter switching circuit, thus greatly reducing transient variations and making it possible to use considerably smaller and therefore less costly switching and filtering elements.
It is a related object and feature of the present invention to provide a preferred embodiment in which the current switched by the transistor switches of the DC--DC converter circuit at no time exceeds the current through either battery and therefore allows the use of less expensive transistor switches.
It is another object and feature of the invention to provide an overload detection circuit for detecting an excessive current in the equalizer circuit which could damage the equalizer circuit components, without consuming significant energy in the overload detection circuit.
Because battery equalizers use transformers with a ferromagnetic core, saturation of the transformer core must be avoided. A very small DC offset, such as a 50% duty cycle in one transistor switched current path and a 49% duty cycle for the other, will cause the operating point on the BH curve to migrate incrementally up or down the curve during each cycle. Traditionally, this problem has been addressed by making the switching control circuit as precisely symmetrical as possible, by providing a small dead-time interval, and by providing snubber circuits to leak off the incremental, unbalance of energy.
It is an object and feature of the present invention to prevent such migration of the operating point on the BH curve of the transformer in a manner which is highly effective because it effectively shorts out the transformer during each off-time interval, and yet does not waste energy because the excess transformer energy is returned into the circuit.
It is another object and feature of the present invention to provide an error signal detecting circuit, for detecting the voltage difference between the voltage on one of the batteries and the average of the voltages on all of the batteries, by a simple circuit which avoids problems ordinarily associated with the floating nature of a battery equalizer circuit with filter inductors in its main battery current conducting paths.